1. Field of the Invention
This invention relates to screen printing apparatus. Specifically, to such apparatus which is used for containing inks and other substances of similar viscosity within the usable area of screen printing frame assemblies, and which utilize a substantially rigid retaining mechanism to secure the apparatus to the screen printing frame.
2. Description of the Prior Art
The screen printing process involves a number of essential set-up procedures. For example, setting up a simple one-color design would include the following:
1. Creating a stencil of the imprinted image on the screen,
2. Securing the screen to a printing press,
3. Registering the stencil in relation to the substrate being printed, and
4. "Taping-off" the interior perimeter of the frame.
To create the stencil, a screen is coated with a photosensitive emulsion and allowed to dry. Once the emulsion has dried, a film positive (reverse of film negative) of the image to be imprinted is placed in intimate contact with the screen, and exposed to light in the UV spectrum. The photosensitive emulsion under the open areas of the film positive cures while the emulsion under the darkened areas of the film positive does not. The uncured emulsion is then washed from the screen using water under high pressure and the screen is allowed to dry. The resulting pattern on the screen is a stencil of the image that will be printed. The screen printing frame assembly (frame and screen) is then attached to a printing press, and aligned such that the imprinted image is printed in the desired location and orientation on the underlying substrate. Following registration, the inside perimeter of the frame is "taped-off".
Taping-off the screen consists of applying pieces of tape around the inside perimeter of the frame. The purpose of applying the tape is to seal off the interface where the screen meets the frame preventing ink from flowing out of the usable area of the screen, and correspondingly into the gap between the frame and the screen. And, since the emulsion is typically coated 1/2" to 1" from the edge of the screen, taping-off the screen prevents ink from flowing through the screen at any open areas around the edge of the screen where emulsion was not coated.
Failure to tape-off screens or properly tape-off screens results in the following adverse condition arising.
1. Misprints due to ink passing through the screen at unintended locations. PA0 2. Excessive ink consumption due to ink flowing out of the usable area of the screen, and correspondingly into the gap between the screen and the frame. PA0 3. Excessive clean-up time since ink between the screen and the frame must be flushed out using solvents. PA0 1. High Flexibility-the screen stretches during printing PA0 2. Solvent Resistant-during printing, the adhesive and the backing are exposed to solvents in the ink PA0 3. High Tack Adhesive-the tapes must not release from the screen during potentially long print runs. PA0 4. Integral Construction-the adhesive must remain with the backing when removed from the screen. PA0 5. Inexpensive-the tape must be reasonably priced since a considerable amount of tape is used on each frame and thrown away after printing is completed. PA0 1. Hazardous to the environment because they are used as a disposable product but are not biodegradable or recyclable. Not only are the tapes not biodegradable or recyclable, but neither is the ink which is on the tape when it is removed from the screen and discarded. In actuality, the inks disposed of with these tapes pose a special concern because some of them contain hazardous chemicals, and if incinerated, can form other hazardous/toxic compounds. PA0 2. Costly due to depletion and the overhead associated with installing/removing the tape to/from the screen. When used disposably, even the most economically priced tapes contribute significantly to overhead. Equally critical is the overhead associated with installing and removing these tapes to/from the screens since the competitiveness of most printers is directly related to minimizing set-up time and maximizing run time. PA0 3. Difficult to handle during installation and messy to handle when removing from the screen. The high tack adhesive makes the tape difficult to handle during application since it tends to aggressively stick to anything it touches. And when it is removed from the screen after printing, the printer has to contend with not only the tacky adhesive, but also with the tape being covered with ink. PA0 4. Unsatisfactory performance over extended periods of exposure to solvent/inks. Regardless of the tape being used, over extended periods of time, the solvents in the inks will attack the adhesive on the tape and reduce their adhesion.
As the term "taping-off" implies, tapes exclusively are presently being used for this application. From the multitude of tapes that are currently available, only those displaying the following characteristics are generally used for this application.
In response to the above requirements, tapes have been developed specifically for this screen printing application. The majority of these tapes are improved versions of existing tapes which provide superior solvent resistance. Tapes termed "split linered" have been developed specifically for taping-off screen printing frames. These tapes get their name from having a removable liner on the adhesive side that has been die cut down the center of the tape width. Corresponding to the line where the liner is die cut, a crease is created in the tape backing thereby providing a fold line. The convenience of this tape is that it allows the printer to neatly and accurately affix the tape to the frame and the screen by removing one liner, positioning and affixing the tape to the screen, and then removing the other liner and affixing the other half of the tape to the frame.
While most printers use generic packaging tapes or solvent resistant tapes, use of split linered tapes is common by large volume printers since their print runs are longer, and they often have the occasion to print the same design several times before making a new stencil. As a result of the special nature of these split linered tapes, their cost is significantly higher than generic packaging tapes or solvent resistant tapes. Whereas a standard clear plastic packaging tape or solvent resistant tape cost approximately $3.00-$4.00 a roll, split linered tape can be as much as 4 times more for the same size roll.
Although printers are currently using a number of different types of tapes to tape-off their screens, perceiving them to be convenient because they are available through numerous sources and perform the task at hand, all tapes suffer from a number of disadvantages when used in this capacity: